The permeability of phenolsulfonphthalein(phenol red), a poorly absorbed drug, was examined as an index of an assessment of gastrointestinal mucosal damage in-vivo. The urinary recovery after oral administration of phenol red was significantly increased in rats with indomethacin-induced ulcers. However, the urinary recovery of phenol red after its intravenous administration was not affected by the ulcers. Gastric absorption of phenol red from the stomach was examined by means of the in-situ loop technique. A significant increase in disappearance of phenol red from the luminal solution was observed in rats orally pretreated with indomethacin. These findings suggest that the increase in urinary recovery of phenol red is due to increased gastrointestinal absorption. This method may be utilized as a simple, useful and non-invasive screening test for an assessment of gastrointestinal mucosal damage in-vivo.